1. Field of the Invention
The present invention relates to an image recording apparatus for recording an image on a sensitive material such as a printing plate mounted on a drum by emitting light beams to the sensitive material.
2. Description of the Related Art
Such an image recording apparatus includes a cylindrical drum for carrying a sensitive material mounted peripherally thereof, a drive mechanism for rotating the drum with a rotary shaft disposed on a drum axis, and a recording head for emitting light beams modulated according to image signals. This image recording apparatus records a desired image on the sensitive material mounted peripherally of the drum by causing the recording head to emit light beams to the sensitive material and moving the recording head in a direction parallel to the rotary shaft of the drum while rotating the drum at high speed.
In this type of image recording apparatus, a sensitive material of varied size is mounted on the drum. The sensitive material is mounted on the drum with opposite ends of the material fixed to the peripheral surface of the drum by using clamping mechanisms. The clamping mechanisms are in an arrangement variable with the size of the sensitive material. Consequently, the center of gravity of the drum is shiftable with the size of the sensitive material mounted peripherally of the drum.
On the other hand, the drum itself must be rotated at high speed for the purpose of high-speed image recording. When the center of gravity of the drum having a sensitive material mounted thereon is not located adjacent the rotary shaft, forced vibration will occur with the drum in high-speed rotation. Such vibration not only baffles high-precision image recording, but could damage the apparatus itself.
In view of the above, Japan Patent No. 2,977,550 has proposed an image recording apparatus having a plurality of counterweights in the form of permanent magnets movably and fixably arranged circumferentially about a rotary shaft of a drum. With this prior image recording apparatus, the center of gravity of the drum with a sensitive material mounted thereon may be located adjacent the rotary shaft of the drum, regardless of the size of the sensitive material mounted peripherally of the drum, by adjusting positions of the counterweights. Thus, the drum may be rotated at high speed without vibrating the drum.
However, the image recording apparatus described in Patent No. 2,977,550, which relies on magnetism for attaching the counterweights to the drum, has a drawback that the counterweights could move in time of image recording.
That is, where, as in the image recording apparatus described in Patent No. 2,977,550, magnetism or other friction mechanism is used to attach counterweights to a drum by a predetermined frictional force, strong torque is required for moving the counterweights relative to the drum to adjust the location of the center of gravity. On the other hand, there occurs a conflicting problem that the counterweights could move when the drum is accelerated or decelerated with a strong force at a start or finish of image recording.
The object of the present invention, therefore, is to provide an image recording apparatus that allows counterweights to be moved by small torque when adjusting the location of the center of gravity, while reliably preventing movement of the counterweights in time of image recording.
The above object is fulfilled, according to the present invention, by an image recording apparatus for recording an image on a sensitive material mounted on a drum by emitting light beams to the sensitive material, comprising a cylindrical drum for supporting the sensitive material mounted peripherally thereof; a drive mechanism for rotating the drum with a rotary shaft disposed on an axis of the drum; guide members formed in opposite end surfaces of the drum and extending circumferentially about the rotary shaft; a pair of counterweights movable along each of the guide members; a moving mechanism for engaging each of the counterweights and moving relative to the drum, thereby to move each counterweight along the guide member; and a fixing mechanism for fixing each counterweight to the guide member in time of image recording.
With this image recording apparatus, when adjusting the location of the center of gravity, the counterweights may be moved along the guide member by small torque. In time of image recording, the counterweights may reliably be held against movement by the action of the fixing mechanism.
In one preferred embodiment, the fixing mechanism includes an engaging device having numerous engaging elements formed circumferentially about the rotary shaft; a positioning device arranged on the pair of counterweights for engaging the engaging elements; and a biasing mechanism for biasing the positioning device toward the engaging device; the moving mechanism moving the positioning device in a direction to disengage from the engaging elements when the counterweights are moved.
Preferably, the engaging device includes an index plate defining numerous engaging bores, acting as the numerous engaging elements, arranged at intervals of fixed angle xcex8 circumferentially about the rotary shaft, the positioning device comprising positioning pins for engaging the engaging bores. This construction is effective to prevent movement of the counterweights.
It is preferred that, in the above construction, the positioning pins are extendible and retractable relative to each counterweight, the number of the positioning pins being n which is an integer of 2 or more, each of the positioning pins being out of phase with the engaging bores by xcex8/n. Then, the counterweights may be positioned with a resolution of angle xcex8/n.
The image recording apparatus may further comprise a sensor for detecting a position relative to the drum of each counterweight fixed by the fixing mechanism. This construction enables a confirmation of the position of each counterweight.
In another aspect of the invention, an image recording apparatus is provided for recording an image on a sensitive material mounted on a drum by emitting light beams to the sensitive material, the apparatus comprising a cylindrical drum for supporting the sensitive material mounted peripherally thereof; a drive mechanism for rotating the drum with a rotary shaft disposed on an axis of the drum; guide members formed in opposite end surfaces of the drum and extending circumferentially about the rotary shaft; a pair of counterweights movable along each of the guide members; a moving mechanism for engaging each of the counterweights and moving relative to the drum, thereby to move each counterweight along the guide member; and a fixing mechanism for fixing each counterweight to the guide member in time of image recording, the fixing mechanism including: an index plate defining numerous engaging bores arranged at intervals of fixed angle xcex8 circumferentially about the rotary shaft; positioning pins arranged on the pair of counterweights for engaging the engaging bores; and a biasing mechanism for biasing the positioning pins toward the engaging bores; the moving mechanism moving the positioning pins in a direction to disengage from the engaging bores when the counterweight is moved.
Other features and advantages of the present invention will be apparent from the following detailed description of the embodiments of the invention.